The rotors in high-speed machinery are often supported by ball or roller bearings where the outer race is held resiliently in the radial direction with damping provided parallel to the resiliency. While this will protect the bearing against high radial loading, it offers no improvement to reduce the axial bearing forces or to contain their peak magnitude if dynamic conditions are present. In many applications, especially in a turbine driven compressor of the radial flow type, the axial load usually controls the bearing life. Accordingly, in order to achieve the requisite bearing life, the operating thrust load acting on the bearing should be controlled and the effect of the dynamic load on the bearing significantly reduced.
It is known that at times it is quite difficult to calculate the axial load at the design phase and to keep it at a low value acting in a predetermined direction. For example, in the case of a radial turbine driven centrifugal compressor, this is so because the net thrust rotor force is the difference of two relatively large forces which can be determined only approximately by integrating the pressures over the impellers in the axial direction. Furthermore, if the inlet and outlet conditions of the turbine and/or its companion compressor change for planned or unplanned reasons, the magnitude or even the direction of the axial thrust load may be changing. Such an unplanned reason may include, for example, the emergency shutdown of the turbine unit due to causes external to itself. During such a shutdown, for axially fixed races, the stresses between the bearing balls and races may easily rise to a peak value, exceeding the yield point, thus causing serious, life-reducing damages. Changing the direction of the axial load in high speed ball or roller bearings could be instantaneously fatal for both the rotor and bearings since in their axially unloaded position, or sufficiently close to it, multiple impacts, usually referred to as chatter, could develop between the balls or rollers and the races causing considerable surface damage.
An important requirement, for long bearing life, among others, is to set the axial thrust to a predetermined low value, and to control its variations closely around this design preload. To illustrate the magnitude of the various forces by ratios, the desirable design axial load action on the bearing might only be 1/5 to 1/10 of the actual uncontrolled axial hydraulic load. The axial load acting on the bearing should be kept within 20 to 30 percent of the design axial load acting on the bearing for safe operation and maximum bearing life. Scalloping the turbine or the compressor wheel is one conventional method for controlling the magnitude and/or direction of the thrust load; however, this method is generally associated with a reduction in efficiency which, especially in the case of compressors, could be significant. Another method of thrust control involves an appropriately sized thrust chamber wherein, most often, a gaseous fluid, usually the working fluid, is introduced and kept at a predetermined pressure by employing a labyrinth seal between its moving surfaces. Such a device with a fluid source of appropriate pressure, can significantly reduce the value between the uncontrolled thrust load and the operating thrust load range. Because of the difficulty in calculating the axial load, however, it is difficult to set the optimum thrust chamber pressure. Further, such a passive system is unable to accommodate the design axial load range.
Furthermore, the problems of bearing support and thrust control also arise with respect to non-rolling bearings such as hydrodynamically or hydrostatically operated journal and thrust bearings. Considering the latter, its load carrying capability has to be designed to carry the maximum thrust load which will occur in the system throughout its operating life. This again may be considerably higher than the steady-state design load. A resilient damped axial suspension with preferably constant load versus displacement characteristics will reduce the dynamic loading in an optimal manner and an active thrust control will reduce the design load. The result of these will be a smaller thrust bearing which is important since in many applications the maximum thrust load and, significantly, the energy loss can be several times higher in the thrust bearing than in the journal bearing. Concerning the journal bearing, it has long been recognized that mounting them in an elastic and damped support will greatly extend their stability limit. However it would be desirable to have improved damping of the bearing mount.
Accordingly it is an object of this invention to provide a support system for bearings supporting a rotating element of high-speed rotating machinery which can act to reduce axial bearing forces as well as dampen radial forces on the bearing and thus serve to extend the life of the bearing.